Cochlear implants of various types have been proposed and constructed. For the purposes of explanation of the present invention, reference will be made to arrangements such as are commercially available from Cochlear Limited. However, it will be appreciated that the present invention is equally applicable to other types of auditory prostheses. An intracochlear electrode array is surgically implanted in a patient, together with a receiver stimulator unit for providing electrical stimuli to selected electrode pairs within the array. The receiver stimulator unit is connected, via an inductive transcutaneous link or a direct percutaneous connection, to an external sound processing device and microphone.
The present invention is concerned principally with the process of selecting appropriate stimuli, and with the actual stimulus pulses delivered in response to the acoustic stimuli. According to known arrangements, stimuli may be applied between different pairs of electrodes, to provide different modes of stimulation. In general, the electrode pair selected is related to the pitch of a detected tone. In this case, stimuli have generally used a consistent pulse timing and shape, and amplitude is determined by reference to the amplitude of the detected sound signal. It is also known to stimulate at a rate related to a detected tone, so as to induce a pitch percept.
However, it has been determined that the perceptions of patients in response to these stimuli are different from the perceptions using the normal hearing mechanisms. It has been determined that, in particular, the response of the auditory nerve to such stimulation is quite dissimilar to the neural response of a normally hearing person to the same sound.
In a paper by Parkins et al, entitled "A fibre sum modulation code for a cochlear prosthesis", Annals of the New York Academy of Sciences, 1983 at p 490, the authors discuss providing stimuli in such a way as to mimic the neural response of a normal hearing person to acoustic stimuli. The stimulus waveform is modified, using a complex mathematical model, so that the post stimulus time histogram approximates that of the normal hearing case. However, the arrangement described is not suited for real time processing so as to facilitate implementation in an implantable or portable device.
U.S. Pat. No. 4,495,384 to Scott et al discloses a real time processing arrangement for a cochlear implant. This disclosure does not describe a system which accounts for the refractory period of nerve fibres, and as a result the stimuli produced do not provide a neural response having a time domain waveform similar to the normal hearing case.
In a paper by Motz and Rattay, (1988), "Signal processing strategies for electrostimulated ear prostheses based in simulated nerve response", the authors discuss the problems associated with hyperpolarisation of the auditory nerve fibres, and consequent loss of perception of higher formants by patients. The stimuli were simulated as if presented from a single electrode. The authors propose the use of further pulses after the initial stimulus pulse, the later pulses having considerable linear increases in amplitude, to improve the perception of higher formants. There is no disclosure of selecting pulses so as to produce a desired post-stimulus time histogram in the auditory nerve structures.
It is an object of the present invention to provide a practical arrangement for generating electrical stimuli so that an auditory nerve response is produced Which better approximates the time domain response of the neural structures of a normal hearing person to a given acoustic stimulus.